Organic Chemistry · Carboxylic Acid Derivatives: Nucleophilic Acyl Substitution Reactions
Polyamides and Polyesters: Step-Growth Polymers
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step-growth polymerization Polymer built by pairwise reactions of any two monomers, usually with loss of a small molecule Full entry → builds polymers from difunctional monomers that react end-to-end, usually releasing a small molecule (typically water) at each bond-forming step — hence the older name, condensation polymerization. Two families dominate: polyamides (nylon-6,6, Kevlar) from diacids plus diamines, and polyesters (PET/Dacron, polycarbonate) from diacids plus diols.
Step growth differs fundamentally from chain growth. Chain-growth polymers (polyethylene, polystyrene) add monomer one unit at a time to a reactive chain end, so long chains appear almost immediately. Step-growth polymers grow slowly — any two species can react — so very high conversion is required for long chains. The Carothers equation DP = 1/(1 − p), relating chain length to extent of reaction Full entry →, DP = 1/(1-p), captures this: reaching a degree of polymerization of 100 demands 99% conversion of every functional group.
Why this matters
- Materials everywhere. PET bottles (recycling code 1), polyester clothing (Dacron, fleece), nylon carpet and rope, Kevlar body armor, Mylar films, and polycarbonate glazing are all step-growth polymers.
- Biomedical devices. Biodegradable polyesters (PLA, PGA) make resorbable sutures, stents, and drug-delivery implants.
- Sustainability. PET depolymerization and enzymatic recycling are active research areas.
The college version
Core Concepts
Monomers and the condensation principle
Step-growth polymers require difunctional monomers: a diacid HOOC–R–COOH with a diamine H2N–R′–NH2 gives a polyamide; a diacid with a diol HO–R″–OH gives a polyester. Each amide or ester link forms by a condensation reaction that releases one molecule of water. The chain is built of alternating A–B units whose ends stay reactive, so growth continues as long as functional groups remain.
Nylon-6,6
Nylon-6,6 comes from adipic acid (HOOC(CH2)4COOH) and hexamethylenediamine (H2N(CH2)6NH2); the "6,6" records six carbons in each monomer. The repeat unit is –[NH(CH2)6NH–CO(CH2)4CO]–. Interchain N–H···O=C hydrogen bonds between amide groups make nylon strong, tough, and drawable into fibers. Nylon-6, the other major nylon, comes from a single monomer (caprolactam) by ring-opening polymerization — step growth in kinetics, without loss of a small molecule.
Kevlar
Kevlar is an aramid Aromatic polyamide such as Kevlar Full entry → (aromatic polyamide) from terephthalic acid and p-phenylenediamine. The rigid, para-substituted aromatic rings force the chains into rod-like conformations that align during liquid-crystalline spinning, giving extreme tensile strength (bullet-resistant vests, ropes, tires) and high heat resistance. Rigidity plus interchain hydrogen bonding — not cross-linking — is the source of Kevlar's strength.
PET (polyethylene terephthalate)
PET forms from terephthalic acid (or dimethyl terephthalate, via transesterification) and ethylene glycol; the repeat unit is –[OCH2CH2O–CO–C6H4–CO]–. Crystallinity gives clear bottles and strong fibers (Dacron), and PET is the most recycled plastic (code 1). Its ester links hydrolyze slowly — the basis of both environmental degradation and emerging enzymatic recycling technology.
Polycarbonate
Polycarbonate forms from bisphenol A and phosgene (or diphenyl carbonate), with carbonate links –O–CO–O–. It is tough and transparent, used for CDs, safety glazing, and water bottles — though concerns about residual bisphenol A have pushed research toward replacements.
The Carothers equation: conversion controls chain length
For a step-growth polymerization with balanced 1:1 stoichiometry, the number-average degree of polymerization is:
DP = 11 - p
where p is the extent of reaction — the fraction of functional groups that have reacted. Implications:
- High molecular weight demands conversion of 99% or more of every functional group.
- Exact 1:1 stoichiometry is essential: excess of one monomer caps the DP because every extra monomer leaves an unpaired end group.
- Removing water (heat, vacuum, nitrogen sweep) drives condensation forward by Le Chatelier's principle, raising p.
How It Works / Step-by-Step Process
Industrial nylon-6,6 synthesis, step by step:
- Combine equimolar adipic acid and hexamethylenediamine — often as the "nylon salt," which guarantees exact 1:1 stoichiometry.
- Heat the mixture; each amide link forms with loss of water.
- Remove water continuously (heat, vacuum, or nitrogen sweep) so equilibrium keeps shifting toward polymer.
- Let the reaction run until p approaches 0.99+, at which point chains average ~100 repeat units.
- Extrude or spin the molten polymer into fibers; drawing aligns the chains and maximizes hydrogen bonding and strength.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Step-growth polymerization | Chain-growth polymerization | Step growth: any species reacts, slow MW growth, needs very high conversion. Chain growth: monomer adds to an active end; chains form quickly. |
| Condensation polymer | Addition polymer | Condensation links release a small molecule; addition links do not. (Nylon-6 from caprolactam is ring-opening step growth with no small molecule lost.) |
| DP | Molar mass | Mn ≈ DP × repeat-unit molar mass (ignoring end groups); related but not the same number. |
| Nylon-6,6 | Nylon-6 | Nylon-6,6 uses two six-carbon monomers; nylon-6 uses one monomer (caprolactam) by ring opening. |
| Kevlar's strength | Cross-linking | Kevlar is not covalently cross-linked; strength comes from aligned rigid chains plus hydrogen bonding. |
| Recycling PET | Unlimited reuse | Recycling usually "downcycles" PET; chemical recycling back to monomers is an active research goal. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine a train built from cars with a hook on one end and a loop on the other; "acid cars" and "amine cars" snap together by spitting out a tiny drop of water. The train only gets long after almost every car has hooked up: if 99 of 100 pairs have hooked, the train is 100 cars long. Building this way, slowly and in any order, is step-growth polymerization.
Worked example
Example 1: Degree of polymerization from the Carothers equation
How long are the chains when 95%, 98%, and 99% of the functional groups have reacted?
DP = 11 - p
- p = 0.95: DP = 1/(1 − 0.95) = 1/0.05 = 20
- p = 0.98: DP = 1/(1 − 0.98) = 1/0.02 = 50
- p = 0.99: DP = 1/(1 − 0.99) = 1/0.01 = 100
Going from 95% to 99% conversion quintuples the chain length — this is why industrial step growth targets ≥99% conversion.
Example 2: Limiting reagent and theoretical yield of nylon-6,6
A reaction uses 10.0 g of adipic acid (M = 146.14 g/mol) and 10.0 g of hexamethylenediamine (M = 116.21 g/mol). The repeat unit C12H22N2O2 has M = 226.32 g/mol (it already accounts for the water lost per link). Which monomer is limiting, and what is the theoretical yield of polymer?
Step 1 — moles of each monomer (n = m/M):
n = mM
n(adipic acid) = 10.0 g146.14 g/mol = 0.0684 mol
n(diamine) = 10.0 g116.21 g/mol = 0.0861 mol
Step 2 — the polymerization is 1:1, so the diacid is limiting (0.0684 mol < 0.0861 mol).
Step 3 — theoretical mass of polymer:
m = n × M = 0.0684 mol × 226.32 g/mol = 15.5 g
Dimensional check: g × (mol/g) × (g/mol) = g. The theoretical yield is 15.5 g, ignoring end groups.
Key takeaways
- Step growth: any two species react; molecular weight grows slowly; needs p → 1 and exact 1:1 stoichiometry.
- Carothers equation: DP = 1/(1 − p); p = 0.99 → DP = 100.
- Condensation polymers release a small molecule (usually water) per bond; chain-growth polymers do not.
- Polyamides: nylon-6,6 (adipic acid + hexamethylenediamine), Kevlar (terephthalic acid + p-phenylenediamine); strength comes from interchain hydrogen bonding.
- Polyesters: PET (terephthalic acid + ethylene glycol), polycarbonate (bisphenol A + phosgene).
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
State the Carothers equation and compute DP for p = 0.98.
Show answer
DP = 1/(1 − p); at p = 0.98, DP = 1/0.02 = 50.
Why does step-growth polymerization require nearly exact 1:1 stoichiometry?
Show answer
With excess of one monomer, chains terminate with two identical end groups that cannot react further, capping the DP below the Carothers limit.
What small molecule is released in the formation of nylon-6,6 and PET?
Show answer
Water (H2O) — one molecule per amide or ester link.
How does Kevlar achieve its strength without covalent cross-links?
Show answer
Rigid para-aromatic monomers form rod-like chains that align during spinning; N–H···O=C hydrogen bonds between aligned chains give extreme tensile strength.
Give one way step growth differs from chain growth in how molecular weight evolves over time.
Show answer
In chain growth, long chains form within seconds and monomer is consumed steadily; in step growth, chains grow slowly throughout and molecular weight rises only as conversion approaches 100%.
Which monomer is limiting when 10.0 g adipic acid reacts with 10.0 g hexamethylenediamine?
Show answer
Adipic acid (0.0684 mol vs 0.0861 mol of diamine); the theoretical yield of nylon-6,6 is 15.5 g.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- step-growth polymerization
- Polymer built by pairwise reactions of any two monomers, usually with loss of a small molecule
- condensation polymer
- Step-growth polymer whose links form with loss of water or another small molecule
- chain-growth polymerization
- Polymer formed by adding monomer one unit at a time to a reactive chain end
- degree of polymerization (DP)
- Average number of monomer units per chain
- Carothers equation
- DP = 1/(1 − p), relating chain length to extent of reaction
- extent of reaction (p)
- Fraction of functional groups that have reacted
- aramid
- Aromatic polyamide such as Kevlar
Sources & references
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